QUESTIONS AND ANSWERS

Radio Record, Volume VII, Issue 8, 1 September 1933, Page 27

QUESTIONS AND ANSWERS

T W. (Petone): A dual wave-trap will . * be described in the August "Radio ‘Times." 2364 (Auckland): I have on hand ° two audio transformers, four valyes-an A442, 415, 409, and a 405a .00015 mfd. and two .0001 mfd. tuning condensers. Has a short-wave set been described which wil incorporate, these components? A.: Yes. The "Sellens" short-wave set, described in the ‘Radio Reeord" dated April 17, 1931. 2. I have an a.c.-6-valve superhet. Would it be as good a dx receiver as a six-valve T.R.F. set? ' A.: Yes-perhaps a little better. Note.-Use 1 mfd. fixed condensers of high test when connecting phones to your set. "Pse" _(Piriaka): I have a two-valve set of the high frequency type. The battery terminals are marked LT-+. HT+1, HT-, LT, HT+2. What batteries and. valves will I need? A.: Do you mean your set employs a high frequency stage, or is a high frequency, i.e., short-wave, set? If the latter, then two 2380 type valves will be suitable.. You will require a 2-volt accumulator and a "B" battery of 90 volts minimum. "HT+1" should be connected to the 30 or 45-volt tapping, and "HT+2" to the maximum voltage G.S. (Timaru): What is the reason for banging on the top side of the dial when volume is turned full on? I have had the condenser plates washed with a special solution and the volume control cleaned, but this stops the noise for only a few days. A.: What do you mean by the "top" side of the dial? It is probable that your trouble is due to a defective volume control, which should be replaced. Af B. (Lower Hutt): I have built the * erystal set described in the 1931 "Guide," and reception with one pair of phones was good. Recently, however, a friend gave me another pair and these, when connected te the set with the others give good reception themselves, but greatly weaken volume of the first pair. A.: The second pair of phones is evidently of much lower resistance than the first pair. Try connecting them in series, i.e., connect one tip of each pair to a phone terminal and connect the other tips together. ,

J ©. (Invercargill): I believe in one * of the early "Guides" a device was described which, when attached to a broadcast set, turned it into a transmitter. | Could you give me further details? A.: You are evidently confusing this with the "Sparrowhawk" short-wave adaptor, described in the 1931 "Radio Guide." The purpose of this device, however, is to permit reception of short-wave stations on a broadcast set, -- (Wadestown) : My reception of outside -statious during the evening is spoilt by morse interference from the wireless station on Tinakori Hills. Can you suggest a remedy? A.: Are you sure the interference is from ZUW? It may bé from a nearby

amateur transmitter. Your pest plan would be to call in someone who understands morse to read the call sign. If the interference is from an amateur transmitter, the only remedy is to approach | the owner of the station and ask for his co-operation, If it is from ZLW, how-ever-and we doubt very much that it is -there is very little you can do, except purchase a more up-to-date receiver. "GREENHORN" (Oamaru): The dial of my set is graduated from 0 to 100. flow can I calculate kilocycles from this? A.: Your best plan would be to make a graph, plotting kilocycles, from 550 to 1500 k.c., against dial readings. Then jot down the numbers at which the principal New Zealand and Australian stations come in, together with the frequencies on which they operate. Plot these points on the graph and join them up with a straight line or a smooth curve. Then if you know either the dial reaiiing or the frequency of a station you can find the unknown quantity by reference to the graph. O C.8. (Hamilton): I have a home- * puilt T.R.F. set employing a 57 as detector, r.c.c. to a 56 first audio. 1 cannot adjust the bias voltage on the 56 correctly. At-present I have a 2000 ohm resistor in the cathode lead, which gives 33 volts bias. I have changed valves and also resistors, but with no better results. The plate voltage on the 56 is 250 volts, applied through the primary of an audio transformer. The voltages have been tested with a high quality 1000 ohms per voltmeter, What is the cause of this oyer-bias?

A.': Byidently some little fault not apparent from your question. According to your figures the 56 is ‘drawing’ roughly 16.5 but if: properly biased it should take only 5 mils. The bias:on a 56 with a plate. voltage of 250 should be 18.5 volts, and from this the value of the ‘bias resistor:in the cathode lead works out at 2700 ohms, You would find that a 8000 ohms resistor would give better results than the one you are using. ILRDM" (Dunedin): I have a Tvalve home-built a.c. receiver, the circuit of which I enclose. What is the approximate drain in milliamps? A.: About 85 mils. 2. I understand the 245 valve takes approximately 34 mils. Does the pushpull "A" class amplifier employed take current equivalent to 1 valve or the total of both? A.: Approximately the total of both. i.e., from 60 to 70 mils. 3. I am troubled with a slight hum. Would the alterations I have shuwn reduee this? A.: You could try the extra smoothing you have shown, but there are several alterations which will probably cure the trouble without resorting to this. First of all connect a .25 mfd. by-pass condenser between the junction of the 2 rf. cathodes and earth. Secondly, try . increasing the by-pass condenser across the bias resistor in the detector cathode lead to .25 mfd. This resistor could also with advantage be reduced to 25,000 ohms. Also try reducing the detector plate feed resistor to 250,000 ohms. URRY" (Palmerston North) and C.D.A. (Feilding): We cannot ad-

vise readers to make drastic alterations to commercial sets, as this should be undertaken only by qualified servicemen. Byen then alterations such as you both suggest are rarely to be recommended. D B, (Ngatira): Where can I obtain * the circuit of a five-valve battery receiver employing two stages of screen grid, r.f. detector, and two audio stages? A.: You. could build the ‘Outspan Five," but unfortunately the issue of the "Radio Record" in which it was deseribed. is completely sold out. You would

information Coupon (fo be used with all requests for . information.) Name Of SEL ....sensscccccenenovess Model WeETeIeIr iii iid Name Tee ee Te SOT SESEOSESEOSHOSOSE . Address PTrrtiriet td PPrrrrTrivrrTiitiititii tid rrr Prrrrrrerrerrer rt tiie Nom de@ plume ..cevenccsoseacese: To be kept in subsequent inquiries Please Note;(1) Be specific ana ovriet, tabvlating, if possible. (2) Write legibly. and on one side of the paper. (8) We do not desigu circuits, (4) Limit three questions uvless 1/- ig enclosed (3) Postal queries limit 3 questions, Inclose stamped and addressed envelope and 1/- fee.

* Two Fine Sets The "EIGHT STAR FIVE" and the "SKY-KING SIX" are described in the August ‘‘Radio Times" Get YOUR Copy Now ! * *

probably find that the "Quality Six," described in the 1983 "Radio Guide," would be quite suited to your needs, . C=. (Dunedin): To our knowledge * devices for reducing power interference are not commercially available in New Zealand, though any dealer would biild one up for you. . ; A C, (Wellington): If I increase the * height of my aerial by 10 feet, will it help to reduce power interference from tramways and power lines? A.: It is impossible to say for certain, for reducing powér interference in this way is largely.a matter of experiment. 2. If not, where could I obtain details" of an aerial system or appliance to remove or-decrease this interference? A.: Line filters for reducing power interference were described in the "‘Radio Record" dated July 21, while a special shielded aerial designed for the same purpose, was described in the following issue. 8. Are the authorities doing anything to. decrease interference? A.: They are doing their best, but it is such a cdlosSal task that it will probably be some yéars before an appreciable improvement is noticed. "(NRONA" (Napier): Would results justify using the -parts suitable in the "Outspan Four" to make up the "Sky-King Six’? A.: Yes, the latter is, of course, a more owerful set. 2, Oould I employ a 34 to1 ratio transformer instead of a 3 to 1, as specified for the "Sky-King Six’’? 4.: Yes. 8. Would this set be fairly selective? A.: Yes, ample selectivity is ensured by the rif. stage. "NFATHS" (Wellington): Could you give me the formula for converting kilocyeles to metres, and vice versa? A.: Kilacycles multiplied by metres equals 300,000, Thus the frequency of a station operating on 500 metres is 300,000 divided by 500, ie., 600 k.c. RBme. (Christchurch) : I have a radio"gram combination set which has developed a fault. When the radiogram switch is lying in its neutral position a crackling noise is heard, but when the switch is turned to either the radio or gramophone this decreases. I replaced the detector yalve, but the set is just as noisy. Neither the tone of the set no? its distance getting properties is affected in any way. ‘The noise still petsists after the removing of the aerial. A.: Most likely a defective switch, which should be replaced. It is also possible that the trouble is due to a noisy valve or resistor. This is clearly a case for a serviceman.